In code division multiple access (CDMA) mobile wireless networks, voice services are predominantly carried over single-carrier radio transmission technology (1xRTT) networks (e.g., circuit switched CDMA networks). Comprehensive emergency service support has been put in place in 1xRTT networks for carrying emergency (e.g., “911”) calls. This includes routing an emergency call to a correct public safety answering point (PSAP) and allowing the PSAP to retrieve a caller's callback number, mobile cell/sector location (e.g., a “phase 1 location”) and a caller's global positioning system (GPS) location (e.g., a “phase 2 location”).
As a quality of service (QoS) becomes available for packet switched CDMA networks (e.g., referred to as high rate packet data (HRPD) Rev. A networks), voice over Internet protocol (VoIP) over HRPD networks becomes possible. Some mobile service providers have deployed packet VoIP over HRPD Rev. A networks using an evolved HRPD (eHRPD) packet architecture. eHRPD provides further enhancements to HRPD Rev. A core network architecture such that eHRPD can support roaming with long term evolution (LTE). IP multimedia subsystem (IMS) is an architecture developed so that IP networks can rapidly deploy new IP-based multimedia services. IMS over eHRPD (e.g., over HRPD Rev. A) will evolve current HRPD networks into IP multi-media networks. The term “eHRPD,” as used herein, may refer to eHRPD over HRPD Rev. A.
For IMS over eHRPD, user equipment (UE) (e.g., mobile communication devices, cell phones, mobile terminals, mobile handsets, personal digital assistants (PDAs), etc.) may be deployed in a dual mode such that the UE can use either a 1xRTT network or an eHRPD network to obtain voice service (e.g., depending on network availability at a particular location). Emergency calls need to be supported in such 1xRTT and eHRPD networks. Due to lack of sufficient standard and the significant time and effort required to implement a native emergency support over eHRDP networks, a generally accepted solution is using the 1xRTT network (e.g., as a “fallback”) to carry an emergency call even when the emergency call is initiated by the UE in an eHRPD mode. One way to perform this “fallback” is to have the UE recognize a call as an emergency call by determining whether dialed digits are a predefined dial string (e.g., such as “911”). Once the UE recognizes an emergency call, instead of allowing the emergency call to proceed over the eHRPD network, the UE will revert back to the 1xRTT network to initiate the emergency call.
Using the 1xRTT network as a fallback works only if the 1xRTT network always has better coverage than the eHRPD network and the 1xRTT network is available for the emergency call. Since 1xRTT networks have been much more extensively deployed than eHRPD networks, a 1xRTT network may typically have better coverage than an eHRPD network. However, there is no guarantee that, for every location of the UE, a 1xRTT network is always available for an emergency call and has better coverage than an available eHRPD network.
For example, an eHRPD network may be available and a 1xRTT network may not be available (e.g., at a particular location and/or time period) due to radio signal complications (e.g., caused by propagation or reflection of radio signals). In another example, during a natural disaster (e.g., since most people have 1xRTT handsets or UEs), the 1xRTT network may be overloaded and unavailable while the eHRPD network may still have capacity. In still another example, due to planned events (e.g., equipment upgrade) or unplanned events (e.g., equipment malfunction), a 1xRTT network may be unavailable while an eHRPD network may not be affected. In these examples, eHRPD UEs will still be able to make normal VoIP calls over the eHRPD network, however, if the UEs rely on falling back to the 1xRTT network for emergency calls, the emergency calls will fail.